First of all, what is burnishing? Well, all it really means is covering the paper with pencil until no more paper shows through, it leaves a nice, subtle sheen to the work that makes it look beautifully finished. The next thing to note is to try NOT to burnish until your last layers. Once your work is burnished you will have flattened out, or filled, all the tooth of the paper with color. It is hard to apply more color over the top of burnished work. You can apply a spray that makes the surface workable again. I have not tried these. I do know that you need to try them out on a test piece first as they can alter colours already put down. There are several techniques that you can use to burnish and some people make it sound like some technique that is really difficult to master, but actually I have found that burnishing happens quite naturally as you build up your layers. In fact, I found out about burnishing completely by accident on my first piece of work. I was happily layering color when all of a sudden I noticed that there was a point when everything magically seemed to blend together and a gorgeous sheen appeared on my work. That prompted me to Google what was happening and I found out it was burnishing. That is still all that I do, but you can do other things. You can use blending tools such as stumps and tortillons to burnish. These simply flatten and smooth out the pencil by the application of pressure. They will not change the colors of your work, you will just add sheen. You can use colorless blenders in the same way, but be aware that they may brighten or darken your colors. You can also simply use pencil pressure to burnish. Applying heavy pressure to the pencil pushes the color deep into the tooth of the paper. I rarely do this because it is very hard on your arm and wrist over time. It works well though, as long as you are certain that you are on your last layers. Don't ever think that there is anything complicated about using colored pencil, you will get beautiful, professional results without worrying about burnishing or blending if you just keep adding light layers of neat strokes.